The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
Photovoltaic (PV) cells that convert sunlight directly into electricity are becoming increasingly important in the world's renewable energy mix. A rapidly developing newcomer to the thin film PV field is based on organic-inorganic perovskite-structured semiconductors, the most common of which is the triiodide (CH3NH3PbI3). Such Perovskites tend to have high charge-carrier mobilities and therefore make ideal photoactive components.
Perovskite solar cells are named after the characterising crystal structure or “perovskite structure”. The photoactive layer consists of perovskite crystals of the very small molecules and ions. This crystal structure is formed rapidly in normal drying processes, and therefore can be difficult to form as a continuous film on a substrate. This is less problematic in typical lab process, particularly when using spin coating to coat a perovskite precursor solution onto a substrate. Spin coating allows for the formation of very homogenous films over an area up to 300 mm in diameter. In a spin coating process, a liquid is applied to a substrate prior to or after acceleration of the substrate to a chosen speed. The substrate spins rapidly and solution on the substrate spreads and dries. Spinning processes quenches crystallization process. Therefore, formation of large size crystals can be minimized or controlled allowing higher coverage to be achieved.
However, it is not possible to use spin coating on a larger scale. Industrial coating process such as slot die coating or any other scalable coating process forms wet film first and then dry the film naturally, by heating or air blowing. This has a significantly different solution dynamic and drying time compared to spin coating. In wet coating, the wet film is susceptible to dewetting, non-homogenous crystal formation and/or the formation of pinholes, all of which have undesirable effects on the function of the photoactive layer. These problems become increasingly more serious the greater the amount and area to which the crystalline material is applied. Resultingly, it is more challenging to form defect free perovskite layers on a given substrate.
It is therefore desirable to provide a new and/or improved process or method of forming a photoactive layer of a perovskite photoactive device such as a solar cell.